The present invention relates to a scanning mechanism which produces a rosette scan pattern providing a sensor field of regard or search volume which is greater than a hemisphere.
While the present invention has particular use in infrared surveillance and tracking, e.g., a linear infrared detector array for infrared search and track (IRST) applications, it is to be understood that the invention is not to be limited to this specific use or form of electromagnetic energy.
Conventional infrared search and track systems use roll-stabilized azimuth scanners to provide contiguous coverage of a limited field-of-regard near the horizon and ahead of the vehicle in which they are housed, such as in an aircraft.
Present applications require surveillance sensors that are capable of scanning over extended fields of regard in a time efficient manner. One current approach, the radial bar scan pattern, is a straightforward approach to satisfying these requirements and provides improved scan efficiency over the roll-stabilized azimuth scanner.
The mechanism for producing a radial bar scan is capable of providing contiguous hyperhemispherical coverage. The scan pattern is generated with a two-gimbal set, which is oscillated about a pitch axis and is stepped about its roll axis. The advantages of the radial bar scan concept are its simple gimbal system, a sensor field-of-view which does not rotate relative to the scan direction, and a constant scan rate. Its primary disadvantages are the singularity point at the center of the scan and the low inherent scan efficiency, because its large gimbals must be stepped in roll and reversed in pitch for every bar. This concept also requires a large optical window to accommodate translation of the aperture in the pitch direction, assuming that the window is located on and rotates with the roll gimbal.
Another approach, the rosette scan pattern, is also capable of providing hyperhemispherical coverage and offers numerous advantages over the radial bar scan pattern and its derivatives, including improved scan efficiency and scan flexibility for different operating modes. Rosette scan patterns currently are generated by a pair of rotating optical elements, each of which deflects incoming rays by the same angle. Conventional deflection elements include optical wedges or prisms, nutating mirrors, or off-center lenses.
Further discussion of these and other scan patterns and the mechanisms for generating them can be found in "The Infrared Handbook", William L. Wolfe and George J. Zissis, editors, prepared by The Infrared Information and Analysis (IRIA) Center, Environmental Research Institute of Michigan and prepared for the Office of Naval Research, Department of the Navy, Washington, D.C., 1978. Because rosette scan patterns are of particular interest in the present invention, reference is directed to the above publication, chapter 22, pages 46 through 56, and a rotating prism scan is described in chapter 10, pages 10 through 12.
There are several disadvantages of using conventional deflection element rosette scanners. One is that the field of view coverage is limited to a hemisphere, and the translation of the optical aperture requires a relatively large viewing window. As scanning progresses through the field of view, optical abberations arise due to defects either in the curvature of the conformal window or from defraction/obscuration at partitions in any segments of the window. Complex optical and/or electronic implementations are required to correct such optical abberations.